1. Technical Field
The present invention relates to a secondary battery and, more particularly, a secondary battery whose electrolyte layer is mainly composed of electrolyte particles, and a method for manufacturing the secondary battery.
2. Related Art
Along with development of portable equipments such as a personal computer and a cellular phone, demand for a small-sized lightweight battery as a power source of the portable equipments shows a drastic increase in recent years.
In particular, it is predicted that a lithium battery realizes a high energy density since lithium has a reduced atomic weight and increased ionization energy. Extensive research has been made in this respect, as a result of which the lithium battery is widely used as a power source of the portable equipments these days.
Expansion of a lithium battery market demands a lithium battery having a higher energy density. In order to comply with such a demand, internal energy of the lithium battery has been made greater by increasing the quantity of an active material contained in the battery.
Concomitant with this trend, a noticeable increase has been made in the quantity of organic solvent contained in an electrolyte (electrolytic solution) which is a flammable material filled in the battery. This results in an increased danger of battery firing and, therefore, the problem of battery safety becomes at issue in recent years.
One of highly effective methods for assuring the safety of a lithium battery is to replace the electrolyte containing the organic solvent with a nonflammable solid electrolyte. Among others, use of a lithium-ion-conducting inorganic solid electrolyte makes it possible to develop an all-solid lithium battery that exhibits improved safety. Active research is now being made in this connection.
As an example, S. D. Jhones and J. R. Akridge, J. Power Sources, 43-44, 505 (1993) discloses an all-solid thin film lithium secondary battery produced by sequentially forming a cathode thin film, an electrolyte thin film and an anode thin film through the use of a deposition apparatus or a sputtering apparatus. It was reported that the thin film lithium secondary battery exhibits superior charge-discharge cycle characteristics of several thousand cycles or more.
With this thin film lithium secondary battery, however, it is impossible for a battery element to retain an electrode active material in a large quantity, thereby making it difficult to obtain a high capacity battery. In order to increase the battery capacity, a great quantity of electrode active materials should be contained in an electrode, for the purpose of which there is a need to construct a bulk type battery.
The bulk type battery is typically manufactured by compression-molding the entire battery element within a mold of a press machine, taking out the battery element from the mold and placing the battery element into a coin type (R621) battery container. However, in the case of the bulk type battery, particularly, an all-solid lithium secondary battery using a sulfide-based lithium-ion-conducting solid electrolyte, it is known that the capacity thereof is reduced by about 7% from its initial capacity when subjected to several cycles of charge-discharge operations at most (see, e.g., DENKI KAGAKU, 66, No. 9 (1998)).
Thus, there is currently a demand for development of a secondary battery (bulk type all-solid secondary battery) having improved performance and being capable of preventing reduction of a battery capacity over the lapse of charge-discharge cycles.